The agreement comes just days before Germany depart for the World Cup in the United States and follows a season in which Undav cemented his status as one of the country's leading forwards.
The 29-year-old enjoyed a prolific 2025/26 campaign, scoring 25 goals and providing 14 assists in 46 appearances across all competitions. His 19 Bundesliga goals also made him the highest-scoring German striker in the top flight this season.
Since arriving in Stuttgart in 2023, Undav has become a key figure both on and off the pitch. He helped the club finish runners-up in the Bundesliga in 2024, played a major role in last year's DFB-Pokal triumph and contributed to Stuttgart securing Champions League qualification for a second time since 2024.
In total, he has scored 46 Bundesliga goals in 86 appearances for the club and has risen to vice-captain during his time with the Swabians.
Stuttgart chief executive Alexander Wehrle hailed the striker's influence after confirming the extension.
"Deniz has been a significant factor in VfB's on-field success over the past three years," Wehrle said.
"On and off the pitch, he has a central role with us and truly identifies with the club. We're delighted that we've agreed terms on an early contract extension with Deniz ahead of the World Cup and we're looking forward to the future with him."
Board member for sport Fabian Wohlgemuth described the agreement as a major boost for both player and club.
"Signing Deniz Undav for another three years is an absolute winner, both for us as a club and for Deniz himself," Wohlgemuth said.
"In his time in Stuttgart, Deniz has become a Germany international with a World Cup around the corner, and VfB finishing runners-up in the league, winning the DFB Cup and just recently qualifying for the Champions League once again has a lot to do with him as well. We're so happy that we've achieved all this in such an atmosphere of mutual trust and appreciation."
Undav, who has scored six goals in eight appearances for Germany, said extending his stay was an easy decision after establishing strong ties with both the club and city.
"I'm over the moon to be continuing my career in Stuttgart," he said.
"This early contract extension means so much to me. The club and the city have become my second home over the past three years â- right from day one, everyone welcomed me and my family with open arms and a lot of love. That's why it's always so crucial for me to repay that faith through my goals and my performances.
"I'm really looking forward to spending many more years as part of the VfB family and enjoying lots more success with the Brustring on my jersey."
Undav heads to the World Cup in fine form after scoring the winner in Germany's 2-1 friendly victory over Ghana in March and contributing two goals and an assist in Sunday's 4-0 win against Finland. Reports in Germany suggest the new deal will also significantly increase his salary, reflecting his growing importance to both club and country.